1. Field of the Invention
The present invention relates to a solid-state imaging device, and more particularly to a solid-state imaging device used for a sensor for detecting an infocus state by a focused light flux from an imaging lens in a single lens reflex type camera.
2. Description of the Prior Art
The in-focus state detection system used in a prior art TTL type in-focus state sensor are divided into two major categories, one using a method in which the in-focus state is detected by detecting sharpness of an image by a focused light flux, by a photo-sensor, that is, a blur image sensing method, and the other using a method in which the in-focus state is detected by calculating the refocusing of an image lens based on a relative positional relationship of two images formed by a special optical system, that is, an image deviation detecting method. The staggered image detecting method can directly detect the defocusing of the image lens and provide a relatively large defocusing signal. Accordingly, it can effectively detect the in-focus state which cannot be detected by the defocused image detecting method.
FIG. 1 schematically shows an optical system in the staggered image detecting method. In FIG. 1, an object image formed on a predetermined image plane 2 by focusing light fluxes A and B from the periphery of an image lens 1 is focused on two line sensors 4-1 and 4-2 each having a plurality of photo-sensors by two secondary focusing lenses 3-1 and 3-2. The outputs from those line sensors are compared to detect the relative positional relationship of the two object images to determine an in-focus state or an out-of-focus state of the image lens 1.
In this method, the outputs from the two line sensors must be equal in the in-focus state. In actuality, however, incident lights to the line sensors is formed by off-axis focused images formed by the image lens and contains comatic aberrations and distortions. Accordingly, in order for the outputs from the two line sensors, that is, light intensity distributions of the two line sensors, to be equal, the optical positions of the image lens, the secondary focusing lenses and the two line sensors must be accurate and precise. Accordingly, the cost increases and practical implementation is difficult to attain.
Several methods for making the light intensity distributions on the two line sensors equal have been proposed. In a method shown in FIG. 2, light shield means 5-1 and 5-2 are arranged in front of secondary focusing lenses 3-1 and 3-2 near a secondary focusing plane to effectively reduce apertures of the secondary focusing lenses 3-1 and 3-2 to attain the equality of the light intensity distributions on the two photosensing planes. In this method, however, if it is desired to efficiently utilize the light applied to the line sensors, the apertures of the light shield members must be adjusted due to differences of aperture F-number values and exit pupil positions of the imaging lenses.
In a single lens reflex type camera, because of various exchangeable lenses, the aperture F-number values of the image lenses vary. As a result, the focused light flux may be shaded depending on the height of the image, as shown in FIG. 3A.
In FIG. 3A, let us consider two light fluxes A1 and A2 focused at two different positions on a primary focusing plane 2. If the aperture of the image lens 1 is large enough to cover both light fluxes A1 and A2, no shading occurs, but if the aperture F-number of the image lens 1 is small, the light flux A1 is shaded.
In order to avoid the above phenomenon, it has been proposed to arrange four sets of secondary focusing lenses 3a, 3b, 3c and 3d and line sensors 4a, 4b, 4c and 4d as shown in FIG. 4 so that they are selectively used depending on the image lens 1. However, this method is expensive because four sets of secondary focusing lenses are used and the arrangement thereof must be adjusted. In order to avoid the above problems, a field lens 5 is placed on the primary focusing plane as shown in FIG. 3B to put the exit pupil of the image lens and the entrance pupil of the secondary focusing lens in a focusing relation to prevent the so-called shading.
As an optical system of a compact and precise focusing device, two line sensors are arranged at a position behind the focusing plane of the image lens at which an object image is formed by the secondary focusing lens having a deflection optical member for monotonously and continuously changing the polarizing angle of incident light for refocusing the object image formed on the focusing plane to measure a light intensity distribution of the object image by means of the secondary focusing system.
An optical system to which an embodiment of the present invention can be applied is briefly explained.
FIGS. 5A and B shows the configuration of an optical system of an in-focus state detecting device to which a solid-state imaging device according to the present invention is applied.
FIG. 6 shows an enlarged view of the solid-state imaging device of the present invention.
In FIGS. 5 and 6, numeral 100 denotes an image lens, numerals 101a, 101b, 101c and 101d denote divided pupil areas, numeral 103 denotes a field lens, numeral 105 denotes a field mask having an aperture 104, numeral 106 denotes a polarizing prism plate which is divided into pupil areas 107a, 107b, 107c and 107d corresponding to the pupil areas 101a, 101b, 101c and 101d of the image lens 100, and numeral 108 denotes a secondary focusing lens. The polarizing prism 106 may have gradually and continuously changing apex angles as shown in FIG. 5B. Numeral 4 denotes a sensor having line sensors 4a, 4b, 4c and 4d attached to a substrate 5.
The operation of the optical system shown in FIG. 5 is now explained.
Light transmitted through the image lens 100 is applied to the polarizing prism 106 through the field mask 105 and the field lens 103. When the image lens 100 is in a defocused state, the light passes through the polarizing prism 106 at different points along a lateral line having different indexes of refraction. Accordingly, a light intensity distribution on the sensor 4 is asymmetric.
Even if the F-number of the image lens 100 varies, the signals from the inner line sensors 4b and 4c of the four line sensors are detected and processed so that shading is avoided. Accordingly, the construction is relatively simple and compact. Further, an intelligent function such as light flux width switching in accordance with a brightness of the object is readily attained.
The F-number mode selection of the image lens may be effected in the following manner. When the exit pupil of the image lens is large, a sufficient light intensity is obtained. In order to reduce a noise component, the line sensor signals from a set A (4a, 4b) and a set B (4c, 4d) are added for each pixel. The line sensors of the set A generate the image signal from the light fluxes transmitted through the pupil areas 101a and 101b of the image lens, and the line sensors in the set B generate the image signal from the light fluxes transmitted through the pupil areas 101c and 101d, and those signals are processed. This is referred to as mode 1.
When the exit pupil of the image lens is small, the light fluxes transmitted through the pupil areas 101b and 101c close to the optical axis on the image lens are utilized in order to prevent shading. Accordingly, the line sensor 4b is used as the set A and the line sensor 4c is used as the set B, and the image signals from those sets of line sensors are processed. This is referred to as mode 2.
When the exit pupil of the image lens is large and the brightness of the object is sufficiently high, the mode 1 processing is not necessary and the light fluxes from the pupil areas 101a and 101d distant from the optical axis are utilized because the light fluxes from the pupil areas 101a and 101d distant from the optical axis include large displacement of the object image when they pass through the image lens and the images are sharp. Thus, the line sensor 4a is used as the set A and the line sensor 4b is used as the set B, and the image signals from those sets of line sensors are processed. This is referred to as mode 3.
In the prior art solid-state imaging device, the mode 1 processing is effected by adding the signals of the imaging device by an external analog signal adder or A/D converting the signals of the line sensors 4a, 4b, 4c and 4d and adding them by a digital adder.
In the former method, however, since the levels of the image signals of the line sensors are relatively low, noises are introduced in transmission lines used to transmit the image signals to the external analog adder so that the S/N ratio is lowered and the exact image signals are not produced. On the other hand, in the latter method, since the signals of the line sensors 4a, 4b, 4c and 4d are A/D converted and added, twice as many as registers for latching the A/D converted signals are required in the mode 1 as compared with the number of registers required in the modes 2 and 3. As a result, a large capacity memory is required. Further, in order to A/D convert the signals of the line sensors, a longer processing time is required than that of the former method in which the signals of the line sensors are added by the external analog adder.
An imaging device which adds signals of two adjacent pixels of one line sensor instead of adding signals of corresponding pixels of a plurality of line sensors is disclosed in Japanese Patent Application Laid-Open No. 80119/1983. It is not relevant to the present invention.